This invention relates to a respiration monitor, in which the respiration activity is monitored by measuring the variations of the thorax impedance of a patient caused by his respiration activity. The monitor includes means for suppressing the indication of disturbing signals, especially those, which are introduced by the heart beat activity, which have amplitudes below a certain threshold value. This threshold value is adjusted to correspond to a predetermined fraction of the amplitude of the measuring signal which corresponds to the respiration activity.
Respiration monitors of this kind are used for monitoring patients especially in intensive care stations. They measure the respiration frequency of the patient, record the respiration curve and indicate respiration irregularities, especially a stand still of the respiration activity (apnea). The impedance variations in the thorax region caused by the heart activity are obscured by interfering influences which are primarily introduced by the heart activity. Not only the respiration activity but also the heart activity result in a periodical change of the thorax impedance, but the amplitude of the impedance variation caused by heart activity is usually substantially smaller than the impedance variation caused by respiration. However, the heart beat frequency is generally higher than the respiration frequency.
In order to suppress disturbances, some known monitors feed the electrical signals obtained by a variation of the thorax impedance to a trigger circuit, which will only deliver an output signal when the amplitude of the supplied input signal exceeds a predetermined threshold value. This threshold value is manually adjustable and is selected so that it is lower than the amplitude of the signals derived by the respiration activity and that it is higher than the amplitude of the signals caused by the heart activity. The disadvantage of this type of respiration monitors is that the threshold value has to be readjusted frequently, as the amplitudes of the respiration signals do not only differ from patient to patient, but may also differ with the same patient over an extended time interval. An additional difficulty is that the threshold value cannot be adjusted accurately, as the periodic impedance variations due to heart activity are generally exceeded by those caused by respiration activity.
In order to avoid such manual readjustment of the trigger threshold, a known type of respiration monitor is provided with a trigger level controller, which automatically adjusts the threshold value to a certain fraction, for example to two thirds, of the actual amplitude of the respiration signal. The readjustment occurs with a certain delay so that it will be primarily influenced by respiration signals having a high amplitude, while it tends not to be influenced by interfering signals which occur between those high amplitude signals.
Furthermore, a lower limit is provided for the threshold value, which is higher than the lowest amplitudes of the respiration signals. However, this lower limit should be higher than the highest possible amplitude of the heart beat signals. In practice, these two requirements can hardly be met simultaneously, as the amplitude of the respiration signals may be equal or smaller than that of the signals introduced by the heart activity. If the lower limit for the threshold value is made as high that it is above the amplitude of the heart signals in all cases, it may happen that the respiration monitor does not respond to weak respiration signals. If the lower limit for the threshold value is low enough for weak respiration signals, the automatic readjustment may fail, if a respiration stand still (apnea) occurs or if the amplitude of the respiration signals is not substantially higher than that of the heart signals. In these cases there will result a threshold value which has a lower amplitude than the heart signals, so that the trigger circuit supplies output signals which are caused by heart activity and which will thus result in wrong indication of the respiration activity. Especially if a respiration stand still will occur, the observer will get the wrong impression of respiration activity.